## anonymous one year ago what is the radius of a circle with the equation x^2+y^2+2x+4y-9=0

1. Nnesha

standard form equation of the circle $\huge\rm (x-h)^2+(y-k)^2=r^2$ where (h,k) is the center point and r =radius

2. Nnesha

so move the -9 to the right side

3. anonymous

do you know what a "perfect square trinomial" is? sometimes also just called a "perfect square"

4. anonymous

Yeah I heard of it.

5. zzr0ck3r

$$x^2+y^2+2x+4y-9=0\\ x^2+y^2+2x+4y=9\\ x^2+2x+y^2+4y=9\\ (x+1)^2-1+y^2+4y=9\\ (x+1)^2+y^2+4y=10$$ Now do something similar with the $$y^2+4y$$.

6. anonymous

So I would get 3 for my answer correct?

7. zzr0ck3r

Now look at what @Nnesha said

8. anonymous

So my answer would be 6?

9. zzr0ck3r

$$x^2+y^2+2x+4y-9=0\\ x^2+y^2+2x+4y=9\\ x^2+2x+y^2+4y=9\\ (x+1)^2-1+y^2+4y=9\\ (x+1)^2+y^2+4y=10\\ (x+1)^2+(y+2)^2-4=10\\ (x+1)^2+(y+2)^2=14\\ (x+1)^2+(y+2)^2=(\sqrt{14})^2$$

10. anonymous

So my answer should be 3.74 correct ?

11. zzr0ck3r

how did you get that?

12. anonymous

I square rooted 14

13. zzr0ck3r

right